Heretofore, iron-based sintered alloys have been used as materials for machinery elements such as engine cam shafts in sliding movement with other members while bearing high surface contact stress. Existent iron-based sintered alloys for use in cam shafts have been generally manufactured by liquid phase sintering using materials of high carbon composition (about 1.5 to 3 mass %). This intends to ensure wear resistance by increasing density and dispersing coarse carbides (grain size of about several μm to several tens μm). Further, since it goes by way of a solid-liquid coexistent state, this also provides a merit capable of diffusion joining of a cam piece and a shaft at the same time with sintering. On the other hand, solid phase sintering has also been used. In this case, there is a method of integrating a cam piece with a shaft by mechanical joining, pipe expansion joining or shrinkage fitting after sintering and heat treatment. Shrinkage fitting is most advantageous for reducing operation burden upon grinding finishing. Existent iron-based sintered alloys of this type can include, for example, those described in Patent Document 1.
[Patent Document 1] Japanese Unexamined Patent Publication No. 63-42357
However, net shaping has been demanded in recent years for parts with sintered alloys. This is because the demand has been increased for simplification of steps, or increase in degree of freedom for profiles, particularly, in cam pieces. For this purpose, existent iron-based sintered alloys involve the following problems. At first, in a case of liquid phase sintering, it involves a problem that shrinkage upon sintering is large and surface flatness is poor. Therefore, grinding finishing is indispensable and it cannot cope with the demand for net shaping. On the other hand, in a case of integration with a shaft by mechanical joining or pipe expansion joining by using solid phase sintering, it involves a problem that concentricity with the shaft is poor. Therefore, grinding finishing for cam profile surfaces cannot be saved after all. Further, coarse grains of carbides are dispersed in the iron-based sintered alloys in this case. Therefore, it results in a problem of high hostility to mating materials in sliding movement. Use of solid phase sintering and shrinkage fitting is advantageous in view of net shaping but it has a problem that hardness cannot be ensured sufficiently. This is because the cam piece is tempered upon shrinkage fitting. Thus, durability is insufficient.